A grain-oriented electrical steel sheet is a soft magnetic material that is industrially used, most commonly, as a material for an iron core incorporated in a transformer, a rotator, a reactor or the like. The features of a grain-oriented electrical steel sheet that are distinct from other soft magnetic materials for iron cores are: that a grain-oriented electrical steel sheet is an iron-base material that has a body-centered cubic crystal structure capable of securing a large magnetic flux density, the magnetic flux density being an index of energy output in a magnetic instrument; and that a grain-oriented electrical steel sheet has a capability to relatively align crystal grains in the orientations in which the crystal grains are most likely to be magnetized, the orientations being expressed, with reference to crystal lattices, as <100> in terms of Miller indices used in the field of physics, as discovered by Honda and Kaya.
Therefore, a grain-oriented electrical steel sheet, though it is a polycrystalline steel sheet, is excellent in the property of being magnetized in specific directions as if it were a monocrystalline steel sheet, and is a material desirable as an industrial product capable of securing a large magnetic flux density as an outcome of a small magnetizing force.
In a grain-oriented electrical steel sheet, the easy magnetization axes of crystals are aligned in specific directions by utilizing the phenomenon generally called secondary recrystallization. The earliest example wherein the above-mentioned concept is disclosed in public as an industrial technology may be U.S. Pat. No. 1,965,559 (1934) applied by P. N. Goss. According to the technology, secondary recrystallization is elicited by dispersing minute particles mainly composed of the compound of manganese and sulfur in a body-centered cubic iron alloy as the second dispersing phase in a steel containing silicon abundantly and by combining cold rolling with annealing.
The features of a secondary-recrystallized structure thus obtained are: that crystal grains, that are generally bound to be several tens to several hundred microns in size, grow up to several millimeters in size and penetrate a steel sheet in the thickness direction; and that the entire steel sheet is covered solely with the extraordinarily grown crystal grains.
A proposal that gives an academic interpretation to such a metallurgical phenomenon is the paper given by May and Turnbull (Trans. Met. Soc., AIME, Vol.212 (1958), P. 769).
According to the paper, in a steel: the original orientations of crystal grains undergo a change by rolling and annealing; the orientations tend to be well arranged relatively in specific orientations under specific conditions; the well arranged orientations have a specific relation with the orientations of crystal grains having <100> orientations coinciding with the rolling direction; by so doing, the nature of the crystal grain boundaries that divide the crystal grains having the well arranged orientations from the crystal grains having <100> orientations is differentiated from that of the other crystal grain boundaries; as a result, the interaction of only the specific dividing grain boundaries with the compounds of Mn and S finely dispersing in the steel reduces; and thus the dividing grain boundaries become likely to move preferentially at a high temperature.
The paper also proposes the above concept by quantitatively expressing it as numerical formulae. In the proposal, with regard to the phase of the finely dispersing compounds, only the size and number thereof are taken into consideration as parameters and the constituent elements thereof are not particularly specified.
If the concept proposed in the paper is valid, it can be said that the second phase finely dispersing in a steel, the second phase being necessary for eliciting secondary recrystallization, may be composed of any material. It may be said that a paper that verifies the above assumption is the research paper written by Matsuoka et al (Tetsu To Hagane, vol. 52 (1966), No. 10, P. 79, P. 82, and Trans. ISIJ, Vol. 7 (1967), P.19).
In the research paper, the authors make the compounds of Ti, C and N, in addition to the compounds of Mn and S, precipitate in a steel, utilize the precipitates as the second dispersing phase that preferentially moves the specific dividing grain boundaries, and thus elicit the secondary recrystallization. Note that May and Turnbull disclose research wherein the compounds of Ti and S are utilized (J. Appl. Phys., Vol. 30, No. 4 (1959), P. 210S).
In the meantime, attempts to improve the magnetic properties of a grain-oriented electrical steel sheet have been continued steadily and Taguchi and Sakakura have invented an industrial product far more excellent in magnetic properties than the invention of P. N. Goss (Japanese Examined Patent Publication No.S33-4710). The gist of the patent is as follows.
In a grain-oriented electrical steel sheet, the orientations, which are expressed as {110}<001> in terms of Miller indices, of the crystal grains are aligned so that the orientations may coincide with the rolling direction. However, the alignment is not perfect and some orientations are dispersed. Taguchi and Sakakura have succeeded in significantly improving the magnetic properties of a grain-oriented electrical steel sheet by greatly reducing the dispersion.
The metallurgical production method employed by Taguchi and Sakakura is largely different from the method employed by P. G. Goss. Whereas P. G. Goss uses mostly the compounds of Mn and S as the second phase finely dispersing in a steel, Taguchi and Sakakura use the compounds of Al and N together with the compounds of Mn and S. On the contrary, by only above measures, the magnetic properties rather deteriorate. To cope with the deterioration of the magnetic properties, whereas P. G. Goss uses a hot-rolled sheet as a raw material, employs two-step cold rolling with annealing applied in between, and controls a final reduction ratio to about 60 to 65%, Taguchi and Sakakura employ single-step heavy rolling at a reduction ratio of about 80% or more. As a result, a high quality grain-oriented electrical steel sheet having a magnetic flux density under a magnetizing force of 800 A/m and a frequency of 50 Hz, namely the value of B8, exceeding 1.88 T has been invented.
The technological difference between the above two inventions is clearly understood when the results obtained by measuring the textures of steel sheets subjected to cold rolling and subsequent decarburizing annealing by the X-ray diffraction method, as shown in FIGS. 1(a) and 1(2), are examined; whereas two groups of {110}<001> and the orientation group wherein the {111} planes are parallel to the rolling plane constitute the main orientations in FIG. 1(a), {111}<112> and the skeleton orientation group ranging from {111}<112> to the orientations close to {100}<012> via {411}<148> constitute the main orientations in FIG. 1(b).
The orientations {110}<001> that cause secondary recrystallization have naturally a different relation with the group of main orientations of a decarburizing-annealed sheet, the main orientations being to be invaded by the orientations {110}<001>. Therefore, it can be estimated that the nature of the grain boundaries that surround {110}<001> orientation grains is different from that of the other grain boundaries and thus the interaction with a minute precipitate phase is also different between them.
Now, the question is, whether the secondary recrystallization by the single-step heavy rolling method employed by Taguchi and Sakakura also depends mainly on the number and size of a minute precipitate phase but does not depend on the constituent elements in the same manner as the secondary recrystallization by the two-step rolling method employed by May and Turnbull.
One of the reasons why the answer to the question is hard to find is presumably that the restrictions relating to the product requirements of a grain-oriented electrical steel sheet tend to suppress the activities on the research and development of the phenomenon. Namely, a grain-oriented electrical steel sheet cannot be regarded as a practically applicable magnetic material merely by filled with secondary-recrystallized {110}<001> orientation grains.
Firstly, a minute precipitate phase that has been utilized for secondary recrystallization must be removed from a steel at the stage of a final product. The reason is that the nature of a magnetizing process is the movement of the domain walls that constitute the boundaries of magnetic domains dispersing finely in a steel sheet, and a minute precipitate phase interacts with the domain walls and thus delays the movement thereof, in other words, deteriorates magnetizing capability.
On the other hand, the single-step heavy rolling method, as it is clear from the nature of the technology, requires a minute precipitate phase more abundant than in the two-step rolling method. Therefore, it is estimated that, in the single-step heavy rolling method, the possibility of requiring more processes for removing the minute precipitate phase after secondary recrystallization arises and, from that viewpoint, the restrictions on the composition of a usable precipitate phase also arise.
Meanwhile, it is known that a minute precipitate phase of MnS or AlN formed by a conventional method reacts with an annealing atmosphere after secondary recrystallization and can be removed easily.
Secondly, a grain-oriented electrical steel sheet is required to have films with a high electrical resistance on the surfaces thereof. The reason for applying the films is that: the use of an electrical steel sheet as an iron core material for electrical machinery and apparatus is based on the induction principle of electromagnetism; in that case, eddy current is inevitably generated in the steel sheet and causes the deterioration of an energy efficiency and, what is worse, sometimes heat is generated in the steel sheet and causes damage to the electrical machinery and apparatus; and therefore it is at least necessary to prevent the eddy current from transferring between the laminated steel sheets for intercepting the above problems to the minimum.
Meanwhile, in a grain-oriented electrical steel sheet produced by a conventional method, films are formed by the reaction of oxides such as MgO, the oxides being used for preventing sticking of steel sheets which is likely to occur because of a high temperature, with steel components when annealing for secondary recrystallization is applied and play the role of the aforementioned films. Further, insulation coating is sometimes applied when subsequent flattening annealing is applied. In that sense, whether or not precipitates are adaptable to such chemical reaction and do not cause a bad influence determines the practicability.
In particular, an insulating material must not be a metal, therefore it must meet with a severe technological standard for securing good adhesiveness with a steel as a coating film, and moreover the severe standard brings about a severe restriction on the composition of a minute precipitate phase for secondary recrystallization.
Now, in the production processes currently used for producing a grain-oriented electrical steel sheet industrially, decarburizing annealing is employed after cold rolling substantially without exception. Carbon is really an element quite unnecessary solely for advancing secondary recrystallization. However, in the method employed by Taguchi and Sakakura, carbon is a steel component necessary for dispersing and precipitating MnS and AlN adjusted at the stage of melting and refining so that MnS and AlN may have the appropriate size and number, in other words, carbon is an element for the preparation of secondary recrystallization and must be removed from a steel before an annealing process for the secondary recrystallization.
Further, in this method, a steel ingot or a slab must be heated to a high temperature of 1,350° C. or higher prior to hot rolling. To avoid such a big burden, Suga et al have invented a new technology disclosed in Japanese Unexamined Patent Publication No. S59-56522. By this method, the necessity of containing carbon in a steel beforehand may be reduced and a decarburizing annealing process may be avoided. However, in this method, nitrogen must be doped into a steel from outside the steel sheet for the duration from cold rolling to before secondary recrystallization annealing, and, as a result, the necessity of introducing an annealing process of a precise atmosphere for controlling the subtle chemical reaction on the surfaces of the steel sheet cannot be avoided.
In conclusion, in prior art, it is difficult to eliminate a decarburizing annealing process, basically unnecessary from the viewpoint of metallurgical principles of secondary recrystallization, or an annealing process as an independent process between a cold rolling process and a secondary recrystallization annealing process.
With regard to this subject, the inventions by Koumo et al, Japanese Unexamined Patent Publication No. S55-73818 for example, should be studied. They have succeeded in producing a secondary-recrystallized steel sheet by applying a conventional method with carbon not contained in the steel at the stage of melting and refining.
However, an annealing process after cold rolling but prior to secondary crystallization annealing cannot be eliminated in actual production. The reason is that it is necessary for forming films required of a grain-oriented electrical steel sheet product to form oxide layers on the surfaces of the steel sheet and make it react to a part of an anti-sticking agent required for secondary recrystallization annealing, and, for doing so, it is technically easy to introduce annealing in a wet atmosphere.
Further, the technology still requires heating a steel ingot or a slab to a high temperature of 1,350° C. or higher prior to hot rolling and thus is still obliged to incur a big burden.
In contrast, as stated above, Matsuoka announced in 1966 through 1967 a secondary recrystallization method wherein precipitates that completely differed from conventional ones, namely TiC, VC, VN, NbC, NbN, ZrC and BN, were used and MnS was not used in the two-step rolling method by Goss.
The technology is an epoch-making one in consideration of the above technological discussions. That is, in the technology, a cold-rolled steel sheet is directly subjected to secondary recrystallization annealing without subjected to decarburizing annealing beforehand and thus secondary-recrystallized grains of {110}<001> orientations fill the entire steel sheet.
In the announcement, though Matsuoka did not disclose the heating temperature of a steel ingot prior to hot rolling, he disclosed that hot-band annealing was applied prior to cold rolling, thereafter cold rolling was applied up to an intermediate sheet thickness, then annealing was applied, and the final cold rolling was finished at a reduction ratio of about 60%.
At that time, the degree of the integration of secondary-recrystallized grains into {110}<001> orientations was evaluated by measuring a magnetic torque in a steel sheet plane, and the results were that most products corresponded to the ones having magnetic flux densities of 1.88 T or less under a magnetizing force of 800 A/m and a frequency of 50 Hz and the products having the state of high grade crystal orientations were few.
Further, the method of Matsuoka is undeniably more complicated than the method of Taguchi and Sakakura or Suga et al and is a technology that cannot make the best use of the advantage of the elimination of decarburizing annealing. Furthermore, Matsuoka did not study even the propriety of the removal of precipitates utilized for film formation and secondary recrystallization required of a grain-oriented electrical steel sheet product and, in that sense, the technology has not reached the level of an inventive technology. In other words, Matsuoka conducted research on secondary recrystallization but did not conduct the development of an electrical steel sheet usable as a practical material.